The invention relates to a new apparatus for the continuous electrochemical desalination of aqueous solutions and is in the form of a wound module.
It has already been known for some time that aqueous solutions can be desalinated by means of electro-dialysis. Appropriate processes and apparatuses are disclosed, for example, in U.S. Pat. No. 2,741,591, U.S. Pat. No. 4,931,160, U.S. Pat. No. 4,956,071,U.S. Pat. No. 4,964,970, EP-B-0 113 387 and desalination 16, 225-233 (1975), Elsevier Scientific Publishing Co., Amsterdam.
According to the known methods, water-impermeable anion and cation exchanger membranes are, as a rule, alternatingly arranged between electrode, which are connected to a direct-current source. The space between two adjacent membranes defines in each case a dilution chamber or a concentration chamber, respectively. If the solution to be desalinated is passed through a dilution chamber, anions are able to migrate under the influence of the electrical potential through the anion exchanger membrane into the adjacent concentrate chamber in the direction of the anode and cations are able to migrate through the cation exchanger membrane into the adjacent concentrate chamber in the direction of the cathode. On the other hand, anions are unable to migrate out of the concentrate chamber through the cation exchanger membrane in the direction of the anode and cations are unable to migrate out of the concentrate chamber through the anion exchanger membrane in the direction of the cathode. As a result of the influence of the electrical potential, a continuous dilution of the dissolved salts in the dilution chambers and continuous concentration in the concentrate chambers is consequently achieved.
It is also known, and in some cases disclosed in the publications mentioned above, that the use of ion exchanger resins in the dilution and/or concentrate chambers contributes to ion exchange and improves conductivity, and that, on the other hand, the ion exchanger resins are regenerated under the action of the electric current. For the purpose of differentiation, electrodialysis using ion exchanger resins is also occasionally referred to as electrodiaresis.
In the known apparatuses, the ion exchanger membranes are usually arranged parallel to one another and to the electrodes in series so as to form a stack. This arrangement has the disadvantages, however, that separate inlet and outlet systems are necessary for every dilution and concentrate chamber and undesirably high current losses occur. In addition, expensive compression devices are needed to seal the membrane stack, but leakage points can nevertheless frequently occur between the dilution chambers and the concentrate chambers.
To avoid these disadvantages, U.S. Pat. No. 4,225413 proposed an electrodialysis apparatus in the form of a wound module in which the anion exchanger membrane and the cation exchanger membrane are wound around a cylindrical, nonconducting core. Wound membrane arrangements had already been proposed for dialysis units in FR-A-2,267,118, U.S. Pat. No. 2,650,709 and GB-A-489,654.
In the electrodialysis apparatus according to U.S. Pat. No. 4,225,413, a central electrode is arrange in the interior of the nonconducting core and the counter-electrode forms the outer casing. The wound membrane arrangement defines a dilution chamber and a concentrate chamber which have approximately spiral cross section and which each have a separate distribution and removal system. The inner ends of the ion exchanger membranes are passed through an opening in the nonconducting core into the interior of the central electrode and are bonded to one another and to the nonconducting core by means of heat or adhesive. The outer ends of the ion exchanger membranes have likewise to be bonded to one another. For this purpose, the ion exchanger membranes have in each case two edges at both ends.
The previously known arrangement is relatively difficult to produce and, in particular, the bonding of various ion exchanger membranes often presents problems and entails an appreciable risk of possible leaks. This is primarily due to the fact that, on the one hand, many of the known ion exchanger membranes continuously have to be kept in the moist state and can therefore only be poorly bonded and on the other hand, ion exchanger membranes which can be processed in the dry state have, as a rule, a certain, troublesome water permeability. It is also troublesome that the dilution chamber and the concentrate chamber form a "hump" over the opening in the nonconducting core, and the achievement of a constant cross section of the chambers and a uniform change in the spacing of the electrodes are made considerably more difficult or even impossible.